Inflammation is thought to play a significant role in the development of the airways hyperreactivity that is a hallmark of human asthma. However, the events causing this inflammation are unknown. Mast cells, and pulmonary alveolar macrophages (PAM), defined as macrophages from the distal airways and alveoli, have been implicated as cells that may initiate the granulocyte influx. Preliminary studies suggest that PAM from atopic asthmatics (but not atopic nonasthmatics) challenged in vivo with specific allergen produce neutrophil chemotactic activity, the identity of which is presently unknown. This is in contrast to data on mast cell activation which demonstrates significant overlap in the levels of histamine found in bronchoalveolar lavage fluid from atopic asthmatics and atopic nonasthmatics after in vivo allergen challenge. From these preliminary data, this proposal will explore the hypothesis that PAM release a chemoattractant substance(s) which causes granulocytes to migrate from the circulation into the airways, leading to increases in airways reactivity. We will use 3 systems: 1) an in vivo human model involving bronchoalveolar lavage (BAL) to identify and purify the PAM granulocyte chemoattractant 2) a rabbit model of allergen induced airways obstruction to supplement the in vivo human studies by addressing time course of granulocyte migration and the effect of pharmacologic interventions and 3) an in vitro system utilizing PAM harvested from human asthmatics to evaluate the mechanisms of PAM activation after allergen challenge and the influence of mast cell products on that activation. PAM will be harvested from human asthmatic and control airways before and after endobronchial allergen challenge using BAL. The PAM will be placed into limited cell culture and evaluated for chemotactic activity which will then be identified and purified. The PAM supernatants and purified chemoattractant(s) will then be instilled into rabbit lungs to determine if they will produce a late asthmatic response. Studies from the rabbit model will be used to supplement the human data. In particular, the time course for the cellular response that occurs after exposure to chemoattractants and the airways response to various experimental chemoattractant antagonists will be evaluated. Histologic verification of the inflammatory response will be obtained. Finally, we will begin to evaluate the cellular mechanisms of PAM activation in response to allergen. We will use an in vitro system which stimulates PAM with allergen in the presence and absence of other cell types, most specifically mast cells or their products. In addition, we will evaluate the effect of allergen priming on a subsequent stimulus to PAM activation, such as that which may be released by the mast cell. It is our hope that the studies outlined in this proposal will identify the factors responsible for initiating the airways inflammation seen in human asthma and lead to specific improvements in therapy.